Effects of Biochar Feedstock and Co-Application with Organic Nitrogen Fertilizer on Soil Carbon Pools and Inorganic Nitrogen Availability

Biochar can increase soil carbon (C) and nutrient retention, but application of raw biochar may reduce inorganic nitrogen (N) availability and crop yields. As a result, biochar is sometimes co-applied with organic fertilizers, though the efficacy of this practice for preventing decreases in inorganic N remains unclear. We examined how applications of hardwood, hay, and softwood biochars (25 Mg ha−1), with and without blood meal co-application, affected C and N dynamics in a Southwest Virginia pasture soil. All biochars increased soil C pools initially and after 11 months. Biochar C accumulated in the coarse particulate organic matter fraction, with hay and softwood biochar C also accumulating in the fine particulate and mineral-associated fractions, respectively. Immediately after application, inorganic N was increased by blood meal co-application, but biochar had no short-term effect on inorganic N. Conversely, nitrate levels were ~81% lower in biochar-amended plots after 11 months, regardless of blood meal co-application and biochar feedstock. These findings suggest that co-application with organic fertilizer may only provide short-term N benefits, highlighting the need to monitor crop yield and N status in seasons following biochar application, as soil test nitrate alone is not a reliable predictor of crop yield in biochar-amended soils.